1. Field of the Invention
The present invention relates to a solid-state camera device of an interline type, and particularly to a solid-state camera device which can suppress and reduce a synchronous noise generated on a reproduced image of an object to be picked up.
2. Description of the Prior Art
In CCD (Charge-coupled device) image sensors of the interline type, signal charges corresponding to respective pixels are transferred to a vertical transfer CCD in a vertical blanking period are read by transferring them into each line of horizontal transfer CCD in synchronism with each blanking period of one field period, so as to obtain an image signal.
On reading the signal charges, since each blanking period is set at a predetermined time in advance, when the number of pixels corresponding to the signal charges to be read is increased, the number of the signal charges to be read in a predetermined time is increased. A high speed reading operation, therefore, is required for increasing the number of the pixels, and, particularly, it is necessary to increase the frequency of a horizontal transfer clock signal to be used as a drive signal for transfer drive of the horizontal transfer CCD.
Accordingly, there are CCD image sensors of this type known by adopting a so-called two-line reading method in which the signal charges are read out by using a horizontal transfer clock signal of a frequency half as large as that of each original horizontal transfer clock signal.
As shown in FIG. 1, the two-line reading method comprises arranging two horizontal CCDs 3a, 3b vertically to plural rows vertical CCD 2 for reading signal charges accumulated in signal charge accumulating portions 1 corresponding to respective pixels in the vertical direction. In this construction, signal charges outputted from each vertical CCD 2 are given to the horizontal CCD 3a, 3b alternately, and each the horizontal CCD 3a, 3b is driven by a frequency which is half (e.g., 37.125 MHz) as high as a frequency (e.g., 74.25 MHz) of a horizontal transfer clock signal. Then, image signals having phases shifting 180.degree. to each other are respectively outputted from the horizontal CCD 3a, 3b through output amplifiers 4, then are read into an external circuit (not shown) alternately, so as to obtain an output image signal the same as those obtained by transfer drive of the horizontal CCD by the original frequency (74.25 MHz).
Incidentally, the horizontal transfer clock signal is formed by dividing a signal generated from an original oscillator 5 into 37.125 MHz by a clock setting circuit 6 and a 1/n counter 7, and is provided to each the horizontal CCD 3a, 3b through a gate circuit 8 and amplifiers 9.
In the two-line reading method, when a frequency by which the signal charges are read is, for example, 74.125 MHz, the nyquist limit as the signal range becomes 37.125 MHz as shown in FIG. 2, which is a half of the above frequency (74.25 MHz). This is the same as the above-mentioned frequency (37.125 MHz) of the horizontal transfer clock signal in the two-line method. A high-frequency component of the clock signal therefore is included in the signal range of the output image signal, so as to make a synchronous noise as shown in the same drawing and generate stripes caused by the noise on a monitor image plane at predetermined intervals. (High-frequency synchronous noises shown in FIG. 2)
To suppress the high-frequency synchronous noise, elimination of the high-frequency component has been tried by passing the output image signal through a low-pass filter (LPF) with a cut-off frequency of, for example, 30 MHz.
However, in the method, the high-frequency component of the image signal is also eliminated as well as the clock signal. Thus, the signal range is reduced to about 80% of the original one, and the definition is deteriorated. While, when the cut-off frequency of low-pass filter is set on the high-frequency side in order to suppress the reduction of signal range, it becomes difficult to sufficiently eliminate the high-frequency component of clock signal, thus the stripes of noise appear on the reproduction image plane.
On the other hand, in the solid-state camera device as shown in FIG. 1, as well as the horizontal transfer clock signal, a low-frequency signal such as a transfer clock signal for the vertical CCD 2 and a one-horizontal-period pulse signal included in the signal range of the output image signal is produced by changing the dividing rates of the 1/n counter 7 for the oscillating frequency from the original oscillator 5 under control of the clock setting circuit 6. Therefore another synchronous noise caused by the counter noise is generated in the output image signal, and thereby appears as stripes of noise on the monitor image plant at irregular intervals.(Low-frequency noise in FIG. 2.)
To eliminate the low-frequency synchronous noise, there is a known method that phases of the output image signal are adjusted and read alternately so that the noise components included in the output image signal for the two horizontal CCD 3a, 3b have reverse phases respectively so as to be cancelled to each other.
However, also in the method, since various frequencies of the low-frequency clock signals are included, it is extremely difficult to eliminate the low-frequency synchronous noise completely. Accordingly, stripes as the synchronous noise appear on the monitor image plane at irregular intervals.
As stated above, when it is attempted to completely eliminate the high-frequency noise caused on reading signal charges by the low-pass filter in the conventional two-line type solid-state camera device, the signal range is narrowed and the definition is degraded. Moreover, when it is attempted to suppress the reduction of the signal range by setting the cut-off frequency of the low-pass filter on the high-frequency side, it becomes difficult to sufficiently suppress the synchronous noise, so as to generate stripes as noise on the monitor image plane.
Accordingly, these methods cannot suppress or eliminate the high-frequency synchronous noise without deterioration of the definition.
While, with respect to the low-frequency synchronous noise, it is extremely difficult to eliminate it by the known cancel method, so that stripes as the low-frequency noise are generated on the monitor image plane.
Thus, in the conventional two-line reading method, stripes of noise caused by the high-frequency and low-frequency synchronous noises appear on a reproduced image so as to degrade the reproduced image.